They are photo ID cards held by individuals who need to work on sites. Their purpose is to ensure that any person entering or working on a site has the appropriate Health & Safety knowledge to work and act in a safe manner. There are various passports and which you require will depend on the type of site you wish to work on. Often individuals who work on different types of site will have to hold a variety of passports.
To obtain a Safety passport you will need to identify the correct one for the type of site you are going to be working on (eg. CCNSG for High Risk Mechanical site or CSCS for Construction or Civil sites etc) and then locate a training provider who is accredited for the passport you need.