What's the purpose of this section in an RF amplifier circuit?
This appears to be your source, or a source - Wideband RF amplifier
Is this your image source? - if not please advise where. 

Their explanation is close to my one in the comments above. I noted that the R2 R3 L2 path provides negative feedback with impedance rising with frequency - which decreases feedback and so increases gain, as they note.

It says

This is a classical RF amplifier design.
The feedback resistors R3 and R2 define the gain.
By changing the values we can get a higher gain at low frequencies, but this will influence the overall gain taper of the amplifier.
Coil L2 is used to get more gain at high frequencies.
If the value is increased, instability will occur. 

My above L2 guess was only partially correct. They say:

Dimensions of L2 are: 6 windings, inner diameter is 3mm and wire thickness must be 0.4mm. In the proto type I used and SMD type for L1, make sure it’s a high Q coil. 





My original assessment:

R2, R3, R4 together provide DC bias.
The spec on L2 MAY mean eg 6 turns wide x 3 layers deep on a 0.4 some-units former - or something else :-). 

Without C3 I'd have guessed it was simply an RF choke to prevent RF feedback BUT as C3 bypasses R2 and as R2 is >> R3 it suggests that it is intended to allow RF feedback controlled by R3 &amp; L2.   

It's negative feedback so would stabilise the gain.
Arguably R3 L2 have a frequency dependant effect with impedance increasing with frequency but it's barn-door wide.