Per Wikipedia:

De minimis is a Latin expression meaning about minimal things, normally in the locutions de minimis non curat praetor ("The praetor does not concern himself with trifles") or de minimis non curat lex ("The law does not concern itself with trifles").[1][2]

In risk assessment it refers to a level of risk that is too small to be concerned with. Some refer to this as a "virtually safe" level

Ancilla is the Latin word for a maid servant or female slave. Hortus is the word for a garden, with the accusative singular form hortum.

So the phrase means "the maid servant (does something to) the garden; the verb is missing from your phrase so we can not know what it is she does. A verb such as curat would make sense: ancilla hortum curat = the maid servant tends the garden.