aller is the infinitive. Aillent is present tense, 3rd person plural of the present subjunctive mood of the verb aller. This topic is not part of the A0 level, so I wouldn't fret over it too much. You'll get there eventually.

Ils ne peuvent pas y aller. -- They can't go there.
Mais il faut qu'ils y aillent! -- But they have to go there!

«aillent» is the subjunctive 3rd person plural conjugation of «aller»; «allaient» is the 3rd person plural conjugation in imparfait. Aller is an irregular verb and its conjugations are  not the same pattern as all other '-er' verbs.  If you are just starting, just know that one of the joys of learning French is dealing with the variety of verb conjugations (tenses etc) and when to use them. This will occupy a lot of your learning efforts, but will fall into place over time. Just start with present tense until the lessons take you further - present tense alone gets you a long way as it is the most common tense of everyday communication. There are many verb conjugation tables available on the web such as - https://www.lawlessfrench.com/verb-conjugations/