There are a few ways to can select multiple assets at a time:
1) Press Ctrl and click on the assets you want to select:
Once multiple items selected, you can use the toolbar to interact with it: Delete, Move, Edit Tags or set them to Live & Expire.
2) Click the check mark when hovering your mouse over the asset:
Hover your mouse over the asset to click the check mark. This is effectively the same as Ctrl + Click.
3) Change your preference to Singe Click to select:
In your Personal Settings: https://app.optisigns.com/app/s/personal-profile
You can check "Single click to select assets" this will allow you to single click to select the asset instead of opening it. Double Click will open the asset in this case.