Guar gum is a seed gum (a so-called “galactomannan”) found in the endosperm of the guar seed, also known as the “cluster bean” which grows in India and Pakistan. Though it’s a very different thing than xanthan gum, which is produced by fermentation, guar gum is used in much the same way and has many of the same benefits. Like xanthan gum it’s about six times as potent as cornstarch for the same job and it doesn’t need heat to activate. You sprinkle it over whatever it is you’re trying to thicken, hot or cold (don’t pour, as guar gum can clump) and apply the whisk. Thickening happens more or less instantly and the texture is smooth and shiny, though not terribly clear. For that reason guar gum is generally recommended for dairy-based gels. Combined with xanthan gum it makes gels of unusual elasticity.
Guar gum can be used as a gluten replacer in bread (see this post for proportions). It also makes a great emulsifier, binder and stabilizer which why it’s found in more than a few commercial products including ice creams where it helps to prevent crystallization. The down side of guar gum is that it’s only so-so in acidic solutions