The ISS communicates with ground assets through a robust network including ground stations and geostationary satellites (TDRSS). However, due to line of sight constraints there are brief gaps in coverage.
(edit - expansion)
These gaps can be seen (at least during the Shuttle era) by looking at the Flight Plan. I've cut-n-pasted the relevant parts of one page from the linked STS-131 Flight Plan to give an idea of how often comm outages occur. Take a look at the link to see what a full page of the Plan looks like. Basically I've cut out all the activity information.
These timeline sections of the Plan show the GMT and Mission Elapsed time at the top, the Day/Night cycles, the orbit count, etc. (The top values are for the shuttle, the bottom ones are for the ISS).
The lines labeled TDRS W (west), E (east), and Z (Zone of Exclusion) show horizontal black bars when the associated Tracking and Data Relay Satellite (TDRS) is in line-of-sight with the shuttle's comm system.
The lines labeled TDRS AVAIL show when any of the satellites are available to the ISS comm system. The gold bar is Ku-band (video) availability, the green bar is S-band (voice and data) availability.
This graph shows the brief periods during which there is loss of voice or video comm due to line-of-sight constraints with the TDRS system.
For more info on the ISS comm system see my answer to this question: Do antennae on the ISS have to constantly move to maintain data links?