The branch of government charged with enforcing (or executing) the laws.
Executing the laws takes the form not only of making sure that laws are followed, but making sure that the legislature's will is carried out. For example, Congress may pass a law that limits the use of ozone-depleting chemicals. However, Congress may not have the time or the expertise to spell out which chemicals count as "ozone-depleting," or how to set what the tolerable levels of the chemicals will be.
Therefore, Congress will delegate that responsibility to one or more federal agencies, who will execute the law by defining the chemicals, setting the levels, and then enforcing them.