What does it mean that a hypothesis must be falsifiable in order to be valid

It means that a hypothesis could only be considered true or false, not from its verifiability, but from its falsifiability

Explanation:

The scientific observation is always oriented previously by a hypothesis to be proven, that is, the science that is based on the inductive method selects the phenomena that will be investigated for the proof of something that is already supposed. For this reason, the verifiability criterion will not always be valid.

Hypotheses that offer no possibility of being refuted through experience should be considered as myths, not as science. To say that a scientific hypothesis must be falsifiable empirically means to say that a scientific theory must offer the possibility of refutation - and, if refuted, should not be considered.

it cools slower in the earth meaning the crystals have more time to form than lava on the surface

geologic time scale (for gradpoint) =)