Extend-o-matic Series A1 Deployable Flag
From Kerbal Space Program Wiki
The Extend-o-matic Series A1 Deployable Flag is an object that can be planted by a Kerbal on EVA, and was added in version 0.20. Known simply as a “flag”, the Extend-o-matic can be customized with designs chosen and made by the player. With 0.21 it is possible to choose a flag prior to every launch. The default flag can be chosen on the start of a new save and can be changed later on the flag pole at the astronaut complex.
Every Kerbal can plant one flag but also can recover planted flags. To plant, simply right-click a Kerbal and click “Plant Flag”. Every flag has a plaque which text can be chosen upon planting.