The term block can have several meanings depending on the context. It may refer to:

Block (CSS)
A block on a webpage is an HTML element that appears on a new line, i.e. underneath the preceding element in a horizontal writing mode, and above the following element (commonly known as a block-level element). For example, p is by default a block-level element, whereas a is an inline element — you can put several links next to one another in your HTML source and they will sit on the same line as one another in the rendered output.
Block (scripting)
In JavaScript, a block is a collection of related statement enclosed in braces ("{}"). For example, you can put a block of statements after an Statements/if...else block, indicating that the interpreter should run the code inside the block if the condition is true, or skip the whole block if the condition is false.