Incorrect spelling, explanation: boundries is an incorrect spelling because the word is a plural noun and its singular form is boundary. As you can see there is an a there, and so should be in plural: boundaries. This is due to the fact, that we create plural forms by adding -s or -es at the end of singular forms (this concerns regular plural nouns). When a noun ends with y this letter changes to i and then -es is added. We do exactly this in boundaries and there is no reason to skip a letter a there.

Correct spelling, explanation: the correct spelling of this word is boundaries because the word is a plural form of boundary. According to the rule of plural regular nouns, when a word ends with y it is changed to i before adding the plural suffix -es. This shows, that in boundary y changes to i plus -es giving us: boundaries. At the same time, boundries is a wrong form, because we do not have any reason to skip a there.

Definition of boundaries:
noun, a limit in behaviour, action, or a sign of a place where something ends and something else begins
It’s okay to say no and set boundaries when you feel uncomfortable.
Tatry mountains serve as a natural geographic boundary between Poland and Slovakia.

Phrases and collocations with boundaries:
set boundaries – show where are one’s limits
Setting healthy boundaries can lead to improved mental health.
Cross boundaries To violate some established rules of behaviour
You should never cross other people’s boundaries.
Social boundaries – Limits that exist in particular social groups, usually based on ethical differences
Historically, social boundaries in the USA have been stronger in the South.