We see Tony Blair at a book signing in a bookshop surrounded by copies 'A Journey'. Blair is signing a copy of his book for a man and asks him what he would like him to put in it (e.g., a dedication or personal message). The man replies 'A sock!'
To understand this cartoon, you firstly need to know the idiom 'to put a sock in it', which is a colloquial, and rude, way of telling someone to be quiet or shut up. According to The Phrase Finder, the sock "was originally a real sock. In the early days of sound reproduction and radio broadcasts the ability to control the volume of sound coming out of the instruments was almost non-existent; sound came out of large uncontrollable horns. However, if a sock was stuffed into the mouth of the horn, then the volume was considerably reduced, hence the saying."
Secondly, you have to be aware of the fact that Blair has been criticised for getting involved in the Labour leadership race by giving his support to David Miliband. He has also been accused of getting in the way of the leadership contest with his book launch, as this cartoon from The Times suggests. Meanwhile, the contents of the book are likely to stir up old divisions within the party. Bearing all that in mind, we can deduce that the man in the cartoon is a Labour supporter (note the flat cap, a working class stereotype) who is unhappy about Blair being the focus of so much media attention at a crucial time for the party.
• Tony Blair: I knew Gordon Brown would be a disaster (The Guardian)
• Tony Blair: Gordon Brown tried to blackmail me (The Daily Telegraph)
• Tony Blair's memoirs go on sale (Daily Mail)