The port on the switch/router will accept a fiber module which you insert into it.
This module will determine the capabilites of the transmission over the fiber you connect it to
QSFP+ is actually four individual 10Gbit connections run over one pair of fiber. It can be bundled to run one 40Gbit link or separated into 4x 10Gbit links
QSFP28 is four individual 28Gbit connections run over one pair of fiber.. It can be bundled to run one 100Gbit link or separated into 4x 25Gbit links.
This means that you either increase transfer speeds between single point to single point connections, or you can get more connections crammed into tight spaces.
Here is a QSFP+ to 4x SFP+ cable from Skylane Optics
I hope this cleared things up!