Existing shares subdivided into more shares at a proportionally lower price each, so a 2-for-1 split turns one ₱200 share into two ₱100 shares. Your holding is worth exactly the same before and after. The point is affordability: a lower price per share means a board lot costs less, which widens the pool of people who can buy in.
See also
See every term in the full PSE investing glossary.