IT'S MINE! - see also Possessive 01 and Possessive 02
On Possessive 01 are the 'mon', 'ma', 'mes'-type words that are attached to nouns (objects, things) to indicate who they belong to. However some of these 'possessive' words aren't very 'specific' - eg 'son échelle' can mean 'his ladder' or, equally, 'her ladder'. So if you want to be a little more exact, you 'tack on' something to clarify exactly whose ladder it is. For this we use the little word 'à' ..... plus a word from set 7 of the pronouns you met on Pronouns 01.
So you might have 'son échelle à lui' ('his ladder') or 'son échelle à elle' (her ladder). A bit long-winded maybe, but that's how it's done.
Perhaps you might want to say 'the ladder is hers" - 'l'échelle est la sienne' - but that's equally ambiguous, so in common usage it's more usual to say 'l'échelle est à elle'. Or (to show a plural) 'the ladders are theirs' (where the people owning the ladders are all male) - 'les échelles sont à eux' - or (where the people owning the ladders are all female) - 'les échelles sont à elles'.
Simplest of all, of course, is where the ladder doesn't get mentioned by name at all, and we have:
||C'est à moi.
||C'est à toi.
||C'est à lui.
||C'est à elle.
||C'est à nous.
||C'est à vous.
|It's theirs (m).
||C'est à eux.
||It's theirs (f).
||C'est à elles.