What is an IFR hold line?

The VFR hold line is the line on the taxi way which vehicles are not allowed to cross without permission from ATC during VFR weather conditions.

The IFR hold line serves the same purposes but during IFR weather.

In most places there will simply be a "hold line" which serves as the hold line during both VFR and IFR conditions.... but in situations where an aircraft waiting at the hold line may interfere with ground-based radio signals (such as an ILS approach signal) an IFR hold line may be positioned farther back from the runway to reduce this risk.