In theory a buyback is price neutral.

The company has less cash in the balance sheet, so its market cap decreases. But there are fewer shares, so the share price is the same.

(This allows hypothetical future growth to disproportionately benefit existing shareholders, but does not intrinsically increase stock price.)

In practice, like another poster pointed out, it signals the company’s belief that its own shares are undervalued, so the market usually increases its estimation of value.