What does it mean if the wavelength is 0.3 m?

Waves are disturbances that transmit energy from one point to another. They usually are depicted as follows (see attached image of a sine wave). Light travels in the form of a wave, and so does sound. In fact, light and sound are different types of waves. Light is a transverse wave - it travels perpendicular to the vibration. Sound is a longitudinal wave - it travels in the direction of the vibration. The sine wave shown can roughly illustrate light as a wave.
These types of waves have repeating patterns. That is, they have periodicity. In the image, if I take the region from 0.0 to 1.0, I basically have captured all the information in the wave, as that particular portion simply repeats throughout. This is this wave's period. The beginning point and the end point of this period are 'identical'. That is, the next repeat in the pattern would simply have the end point of the first one as the first point in the next. Wavelength is simply the distance between these two identical points. It is technically defined as the distance between two crests or trough - or the highest and lowest point in waves - but for repeating waves like a sine wave shown it can be the distance between any equivalent point in the wave.
A wave with a wavelength of 0.3m means that the distance between crests (or any equivalent point) is 0.3 meter. For comparison, the visible light has a wavelength in the range of 390nm to 700nm (red to violet). Meanwhile, sound waves are from 17 millimeter to 17 meters. 
http://www.qrg.northwestern.edu/projects/vss/docs/communications/1-what-is-wavelength.html

https://www.acs.psu.edu/drussell/Demos/waves-intro/waves-intro.html

https://www.bbc.co.uk/bitesize/guides/zs86v9q/revision/2