Board Thread:Questions and Answers/@comment-70.195.133.100-20150106160845/@comment-6697950-20150106164804

It means that every 50 levels (at least for the One-handed Sword), a new mod slot opens up and you can add an additional mod to the skill. At level 50 you can have 1 mod. At level 100 you can have 2 mods. At level 150 you can have 3 mods etc.