A marine layer is an air mass which develops over the surface of a large body of water such as the ocean or large lake in the presence of a temperature inversion.
In the case of coastal California, the offshore marine layer is typically propelled inland by a pressure gradient which develops as a result of intense heating inland,
blanketing coastal communities in cooler air which, if saturated, also contains fog.
The fog can last until midday when the heat of the sun is frequently strong enough to evaporate it. An approaching frontal system or trough can also drive the marine layer onshore.

Source - Wikpiedia