Complex sentence: When users visit a website matching a script in their personal script library, Greasemonkey invokes the relevant scripts, which can modify a webpage in any way which JavaScript allows--with certain Greasemonkey security restrictions.
Split into simpler sentences: When users visit a website matching a script in their personal script library, Greasemonkey invokes the relevant scripts. Greasemonkey scripts can modify a webpage in any way which JavaScript allows, with certain Greasemonkey security restrictions.