Two fifths of the score of a beer on a BJCP Scoresheet is Flavor. If you consider that Aroma is a large component of Flavor, it's really 64 percent. However for Scoresheet purposes, it's 20 points out of the total 50. The hints on the scoresheet remind you to comment on malt, hops, fermentation characteristics, balance, finish/aftertaste, and other flavor characteristics. Let's consider each in turn.
Malt in general, since barley was traditionally used in baking, is going to have many similar flavor components as bread. Depending on the roasting level, it may be bready, toasty, roasty, caramel, toffee-like, or even have dark fruit characteristics (most commonly plums, prunes, or even cherry). Some malts like pilsner, may have a slightly sweet corn aroma due to the precursors of DiMethyl Sulfide being present (which can smell like cooked corn). If it's allowable for style, it will say so in the Style Guidelines. Some less desirable characteristics may be burnt or grainy flavors (although in certain styles they may be allowable or even desirable).
Broad hop flavors are generally the same as their aromas. Citrus, floral, spicy, herbal, earthy. grassy, piney, and woody are common descriptors. You should try to differentiate though and describe it more specifically whenever possible. For example, are you tasting orange, lemon, or grapefruit when you say citrusy? Some new hop varieties will even have tropical fruit flavors like Mango or Pineapple. Sometimes the hop flavor will detract from the Flavor. Too much hop mass can sometimes be too grassy or contribute a vegetal flavor. Or old hops may even give a musty or cheesy flavor. Bitterness is also a hop contribution, so this will play into the balance and aftertaste.
Fermentation characteristics are the yeast's contribution. Ale yeasts will sometimes contribute fruitiness to a beer. This may range from apples or pears to bananas, berries, citrus (like lemons in saisons), raisins, grapes, or stone fruit (think pitted fruit like peaches). Some lager yeasts will have components that are present but need time and conditioning to age out (like diacetyl or sulfur components). The Style Guidelines will tell you what is or isn't appropriate for style, so you can score accordingly.
Balance, Finish, and Aftertaste are closely related and extremely important considerations for most styles. Some styles like the Scottish styles only have enough hop flavor to keep the beer from being overly sweet. Others such as IPAs should be balanced more to the bitter side with the malt only contributing enough to keep it from being a harsh bitterness. How the Flavor finishes and lingers is also part of this. It is dry and crisp with a slight lingering bitterness? Does it finish with warming alcohol? Or does it finish with a burning hot alcohol? Again, these are mainly important for whether they're appropriate for the style.
Finally, Other Flavor Characteristics are the catch all for anything else you taste. Most of these are going to be off flavors like Acetaldehyde (a sour green apple flavor), hot alcohols (fusels), astringency (harsh drying like sucking on a tea bag), or oxidation (like wet paper or cardboard). Solvents (acetone or paint thinner) or Phenolics (ranges from smoky, spicy, peppery to plastic, band-aid, or medicinal) are other notable flavors that may or may not be off flavors. Sometimes there may even be metallic flavors (tin, copper, iron, or even blood-like) or meaty flavors (beef broth).
In summary, there are a lot of potential flavors you may encounter in beer. It all boils down to whether they're appropriate for style at the levels that are present. Pay close attention to the Style Guidelines for what levels are required, appropriate, allowable, or inappropriate. If the Guidelines say it's required at a certain level, score it accordingly if it's either right on or out of range. If it's allowable, don't ding a beer if it isn't there. If it is inappropriate, score accordingly if it is there.