The ‘turning of the tide’ is literally the change of the tide from incoming to outgoing, or vice-versa. Normally the phrase is used to denote some change from a previously stable course of events.

From Shakespeare’s Henry V, 1598:

Hostess:
Nay, sure, he’s not in hell: he’s in Arthur’s bosom, if ever man went to Arthur’s bosom. A’ made a finer end and went away an it had been any christom child; a’ parted even just between twelve and one, even at the turning o’ the tide: for after I saw him fumble with the sheets and play with flowers and smile upon his fingers’ ends, I knew there was but one way; for his nose was as sharp as a pen, and a’ babbled of green fields.