It means that when they meet the Muslims and greet them, they display their Imaan and friendship to them. However, as soon as they return to their leaders and are alone with them, they confess (to them) that they were truly on their religion (of shirk), but they were only making fun (and mockery) of the Muslims by showing that they were also Muslims.
In the above Verse, the words, ‘and when they are alone with their Shaitans (Satans)’ mean when they are alone with their leaders from among the Jews, the Mushrikeen (polytheists) and the Munafiqeen (hypocrites). (Tafseer Ibn Katheer from Suddi Vol.1 page 51). Some commentators have stated that it means when they (the hypocrites and mushrikeen) are alone with their friends who are hypocrites and Mushrikeen, they inform them that they were only making fun of the Muslims.
Here, in this verse, Allah Has used the word ‘Shaitan’ (Satan) to refer to the leaders of the unbelievers and also those who are hypocrites and Mushrikeen. This word has been used for them on account of their wicked nature and their rebellious attitude to the religion of Islam. In this regard, Hafiz Ibn Jareer (A.R.) has stated, ‘Every rebellious, insolent and arrogant person or thing is from among the Shaitans, and a Shaitan (Satan) is from among men or Jinn’. It is based upon this meaning, the leaders of the Unbelievers, as well as the laymen who are hypocrites and mushrikeen have been called shayateen (satans). (Tafseer Ibn Katheer Vol.1 page 51).