Feeler strips are used in much the same way as feeler gauge blades.
They are thin strips of machined and polished steel which are available in various sizes.
The thickness is marked on each feeler strip for easy identification and double ended use.
Feeler gauge strips cover the same range of sizes as standard feeler gauge sets.
Advantages
Feeler strips are more disposable than gauge sets. They are often worn out by regular use, but can be replaced at low cost.
If you know a clearance needs to be a certain size, then you can simply buy the feeler strip which is the correct size and alter parts either side accordingly.
Disadvantages
Buying individual strips may cost more than purchasing a feeler gauge set. It also may be more difficult to store the feeler strips without them becoming bent or damaged when not in use.