Hayley’s client wants to drive sales of her new cookbook that’s about gluten–free desserts. If Hayley sets up a Display Network campaign that targets potential customers using keywords and topics and the “Target and bid” setting, her client’s ads can show when:
the keywords and bid match
the keywords and topics match
only the keywords match
only the topics match
If you target keywords and topics in the same ad group, your keywords will be considered first when Google decides where to show your ads. Your ads won’t be eligible to appear on pages that don’t match your keywords — even if these pages are related to the topics that you’ve chosen.
Restricts your ads to showing for the targeting method you’ve selected, like keywords.
Read more here: https://support.google.com/adwords/answer/1209882?hl=en