High impression share and low conversion rate/low conversions
This time slot may not be the most efficient time to show ads and spend money, because even though ads are showing, they are not converting as well as they do on other occasions. Impression share is probably high because your competitors are not bidding at this time.
Low impression share and high conversion rate/high conversions
The traffic you’re getting at this time slot is converting at a higher rate than other time slots. However, you’re not showing enough and are losing impression share. Increase bids to show your ad more often. If the campaign you’re looking at is maxing out on a budget, increase the budget.
If you’re not tracking conversions, CTR is the next best indicator of performance.