Turnover of 100% implies the whole portfolio was replaced during the year. A broad index fund may run under 5%; an active fund 60% to 120%; a systematic momentum strategy several hundred percent.
Each unit of turnover carries commission, spread and market impact, none of which appear in the total-expense-ratio. As a rough guide, 100% turnover in liquid large caps might cost 0.2% to 0.4% a year, and considerably more in small or illiquid names.
In taxable accounts turnover also converts unrealised gains into realised ones, accelerating tax. High turnover is not automatically bad, but it must be paid for out of the strategy's gross edge before anything reaches the investor. See slippage and fee-drag.
Related: slippage, fee-drag, total-expense-ratio, tax-loss-harvesting, momentum-factor, equal-weighted-index