Number (1) is just the regular pump in the boiler that circulates water, not an electrically controlled valve as the label says.

So, when you want the boiler to supply heat: energize the boiler pump(1), deenergize the solar tank pump(3).   Boiler heated water flows from the boiler, through the floor loops (on left of diagram), and back to the boiler.

When you want heat from the solar tank: energize the solar pump (3), deengergize the boiler pump (1).  Water then flows from the solar tank, through pump 3, out through the floor loops, and back to the solar tank via the top line.

Check valve (2) prevents the solar pump from pushing water through the boiler in the wrong direction when in boiler mode.  The check built into pump (3) prevents the boiler from pushing water back through pump (3) and into the solar tank in the wrong direction when in solar mode.

Maybe its confusing that the floor loops are not completely shown?  The four floor loop lines on the bottom left of the diagram go around and connect to the 4 lines coming into the top left of the diagram.