I have not used it but it appears to be primarily a wine additive extracted from egg whites.
Here's a quote from Norther Brewer:
"When added at the rate of 1 gram/gallon, lysozyme will stop malolactic fermentation in wine. Some winemakers use half the normal dose instead of sulfite to kill naturally-occuring malolactic bacteria in red grape musts; if you later fine the wine with Bentonite, this will remove the lysozyme, and you can then add a cultured malolactic strain."
For beer I believe it is used to help kill off lactic acid bacteria that could sour or spoil the beer. It is basically an anti-bacterial additive as it attacks bacteria but is not harmful to yeast.