This is the syntax for a basic gallery of images:

<gallery caption="Whole Gallery">
File:Example.jpg |Caption for individual image |alt=alt language
File:Example.jpg |Caption for individual image |alt=alt language
File:Example.jpg |Caption for individual image |alt=alt language
<gallery>

The "Whole Gallery" typically appears in bold-face type above the images. The "Caption for individual image" appears underneath that individual image.

Both of these things are called captions, which is confusing. ("Just change the caption". "No, I want to change that thing at the top, not the captions!") It might be better to call the whole-gallery "caption" a "title" (or perhaps a "label"). The syntax could change to reflect this:

<gallery title="Whole Gallery">
File:Example.jpg |Caption for individual image |alt=alt language
File:Example.jpg |Caption for individual image |alt=alt language
File:Example.jpg |Caption for individual image |alt=alt language
<gallery>