Multisync monitors is the name given to any monitor that can display a range of frequencies, as apposed to a fixed frequency monitor.
15-31Khz Multisync Monitors
Obviously, all PC Monitors are Multisync monitors as they can display (usually) a large range of resolutions. PC Monitors typically can display modes with horizontal syncs of 31Khz - 100Khz. For reference, Standard arcade displays are 15Khz, Medium Res 25Khz (a small number of games, like 720 degrees) and Hi Res is 31Khz, which is the same as VGA.
However, in this (arcade building) context, 'Multisync' monitors are typically used to refer to the much smaller number of monitors that are capable of displaying 15 - 31Khz signals. i.e a range that lets them display native arcade signals.
The main advantages of such a monitor are:
1) Can display virtually all arcade games at the intended native resolution
2) Can display a modern-OS happy 31Khz (or higher) VGA signal - so your OS/Frontend/PC Games/Other Emus can all run normally on the same screen
Common (available) examples of these 'Multisync' monitors:
Betson Imperial/Kortek KT-2914(Review: )
Wells-Gardner D9200 (Review: )
Wei-Ya M2929DC-62 / BilLABS BL27CA1K/BL27CA1Q (Review: )
These are open-frame monitors supporting 15-25-31Khz, (up to 640x480 VGA). I believe some D9200's and Kortek can also display 800x600. The Wei-Ya is in fact a 15-50Khz, so can display up to 1024x768.
(Older) 15-50Khz(ish) Presentation Monitors
There are some (no longer made) multi-sync monitors, typically made by Hantarex or NEC, such as a NEC XM29, that also support the 15Khz, but also support Higher resolutions such as 1024x768. They typically support signals from 15Khz to 50Khz. They used to cost an absolute fortune new, but can often be picked up cheaply due to the move to lighter flat screens.
BYOAC thread on NEC XM29plus: 
Large listing of multi-sync presentation monitor frequencies & specifications: 
Additional multi-sync presentation monitor information: 
The new Wei-Ya (see above) has similar capabilities. These older Presenation monitors tend to be cased, not open frame.