Execute at

The at codeblock compiles into /execute at. In the following syntax examples, @FULL_SELECTOR is a valid entity selector, COORDS is a valid Minecraft coordinate set, and ANCHOR is either eyes or feet.

At Selector

at @FULL_SELECTOR ANCHOR
    ...
end

At Coords

at ANCHOR COORDS
    ...
end

At Relative to Selector

at @FULL_SELECTOR ANCHOR COORDS
    ...
end

At Vector

...

At Vector Scale

...

See this minecraft.wiki page for more details.