Following @Luke Sawczak’s nice comment the sentence

On a toujours été…

is the past form (one of them) of the sentence

On est toujours…(present tense)

In particular, this is the so-called passé composé (perfect tense in French learning books for Anglophones) which is composed as follows:

auxiliary verb (avoir or être)+past participle (participe passé) of
the conjugated verb.

Hence

On est==>On a (avoir conjugated) été (participe passé of the verb
être)

On est ==> On a été

which may interpreted according to context by various ways in English, including

one is/people are/we are/they are ==> one has been/people have been/we have/they have been or one was/people were/we were/they were

See here for the personal subject pronoun on or even better here (not for beginners).

There are three major groups of verbs in French; the first two regular. The third one contains the so-called irregular verbs. Nevertheless, one can split this group into sub-groups to facilitate learning. One of the major tasks of a beginner is to learn how to conjugate the verbs of the third group and how to learn the past participle of them. The first two are not hard at all.
See here for more.

As in many other languages (English, German, Romanian) the verb être (to be) is highly irregular in terms of conjugation and constructing the past participle (été).

Last but not least French language is full of homographes (and homonymes more general). You have just encounter one homographe (été = summer but été = been).
See here for more about this subject.

I suggest also you use an online dictionary. For instance

https://www.larousse.fr/dictionnaires/francais/%C3%A9t%C3%A9/31337/homonyme