Translation:I will read the letter.
Yeah, in order to say you're actively doing something right now, you have to convert the verb into て-form (which Duolingo hasn't covered yet) and add います, so it'd be てがみを読んでいます。Very useful but usually a later lesson, since て-form is different for every consonant and takes some memorization.
I'll answer what I understand about this. If someone understands this better please correct me or confirm what I'm saying!
手紙は読みます (tegami wa yomimasu) would be "I read letters", as in, generally speaking, you read letters. You know how to read letters. You read letters often. Etc. As opposed to 手紙を読みます (tegami wo yomimasu), which would be "I read a letter" or "I read the letter", in the sense that you are reading an specific letter in this moment.
If I understand your question...
読む is the plain form of the verb and the verb is often conjugated (or in plain speak, the last kana is often changed) to give it different meanings. When you conjugate a verb like 読む to 読み, one of the possible meanings that you have is "verb, and" so you would use it in long lists of various actions if the order of the verbs doesn't matter. It does sound a tad bit stiff to me though when this is used. Take for example
彼は公園で遊び、休みました。(kare wa kouen de asobi yasumimashita/He played at the park and rested).