Mum and secret man

So I first saw that my mum had been texting this man called "billy boy" time passed and she started acting weird, I know I shouldn't have but I went down my mums phone and saw these messages which were something I did not want to read, but I saw that she had been round his and lied to me and my brother about it, I don't know what sort relationship this is and why she is keeping it secret any advice