What is this glow over some players' weapons?

The glow effect is known as a sheen, and is visible on weapons which have had a Specialized Killstreak Kit applied to them.

From the wiki (Two Cities link also added for context/further reading):

The next tier of Killstreak Kits. In addition to a kill counter, a Specialized Kit also applies a colored sheen to the weapon. As the player's streak increases, the sheen becomes more intense. Specialized Kits are obtained by completing Specialized Killstreak Kit Fabricators, which can be found as a random reward from completing Operation Two Cities.

The following sheen colors are available:

• Team Shine (red or blue, matching the player's team)
• Hot Rod (pink)
• Manndarin (orange)
• Deadly Daffodil (yellow)
• Mean Green (yellowish-green)
• Agonizing Emerald (blueish-green)
• Villainous Violet (purple)

Professional kits also include the sheen (and the kill counter from any kind of kit) along with a special effect placed on the character model's eyes.