Active risk relates to how large the active positions the investment manager has are relative to a benchmark. A perfect index-tracking fund will hold all the same stocks in all the same proportions as the benchmark – its active risk is zero. Every time the investment manager decides to not hold, underweight or overweight a stock relative to the benchmark, the active risk increases. BUT remember that doesn’t mean the fund is riskier in the real world. If the manager chose to not invest in any company that borrowed money or wasn’t backed by real assets, their active risk would be large and the risk of them underperforming the benchmark would be higher but the risk of you losing money would be far less.
Active share is a measure of active risk.