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This video better explains what rasterizing does. Essentially it means you can no longer edit the layer. So if you're going to rasterize a layer of text, make sure you check for spelling errors FIRST as you won't be able to edit the text after rasterizing, and you would need to make a new text layer and re-type the text.
Make sure you create a duplicate of the layer BEFORE you rasterize (press CTRL and J on your keyboard).
To rasterize a layer/s. Right click on the layer in the layers menu in Photoshop and choose 'rasterize layer' you can select multiple layers at once by holding down the CTRL key on your keyboard, then clicking on all of the layers you want to rasterize at once.
I mainly rasterize layers when making seamless repeating patterns in Photoshop, or if I have a bunch of layers and don't want them taking up a lot of room in the layers menu.
Choose ok when this menu appears: