A complex sentence is a sentence consisting of an independent clause and a subordinating clause. Or, to put it more simply, complex sentences involve more than just a straightforward noun + verb. Complex sentences include extra phrases added to a base sentence, but which cannot stand alone as a sentence. For example:
Published 4:00 am, Thursday, January 9, 1997. 1997-01-09 04:00:00 PDT Yosemite National Park -- Yosemite Valley is slowly drying out after what park rangers are calling the worst flood here in recorded history, leaving behind hundreds of upturned tent-cabins and acres of wrecked camping sites.
There are lots of swimming spots in spring and summer in Yosemite Valley along the Merced River. One of my favorites is the sandy beach near Sentinel Bridge, with its lovely view of Half Dome. When the water level is high enough you can also swim in Tenaya Creek and Mirror Lake.