When que is used with tener, the meaning of the verb becomes "must." Tengo que leer = I must read, Tienes que soñar = You must dream, Tenemos que caminar = We must walk, etc.
Leer is the infinitive form of the verb. It is roughly equivalent to "to read" in English. Though in certain contexts, it is better translated as "reading". Leo is the conjugation of leer that specifically means "I read".
Below are some links on Spanish verb conjugation. You may notice that all of them include the vosotros(as) form, which is generally only used in Spain. Duolingo does accept those answers, as well as answers in the vos form that is used in Argentina, if you would like to practice those. It doesn't emphasis or teach them though since the focus here is on Spanish spoken commonly throughout Latin America. Learning those forms is fairly optional in terms of being understood.
Introduction to Spanish Verb Conjugation https://www.thoughtco.com/introduction-to-spanish-verb-conjugation-3079157
Spanish Present Tense Forms https://www.spanishdict.com/guide/spanish-present-tense-forms/
Beginners Spanish Podcast 5 – Conjugating Spanish Verbs (The people who does this series are from the UK and Spain, so that may or may not be confusing for you. As someone from the States, it can sometimes feel like double-translating for me—US English to UK English to Spanish—but I've found it worthwhile nevertheless.) https://lightspeedspanish.co.uk/20110908-free-spanish-podcast-5-conjugating-spanish-verbs/
FULL DISCLOSURE: Native English speaker - US, Southern Appalachian dialect. Other uses of English may vary. Advice about Spanish should be taken with a grain of salt.