Prandtl number signifies the thickness of thermal boundary layer and thickness of hydrodyanamic boundary layer, depending on whether it is equal to one, or more than one or less than one. If it is equal to one, it signifies that thickness of thermal boundary layer is equal to that of velocity boundary layer. Hence it the ratio of momentum diffusivity to thermal diffusivity.

Significance -

If Prandtl number is small, it tells us that thermal diffusion is dominant in comparison to momentum diffusion. That is for a given fluid flow problem, the flow conditions remaining the same, if we want higher heat transfer rate we have to use a fluid that has lower Prandtl number.

http://nptel.ac.in/courses/Webcourse-contents/IISc-BANG/Heat%20and%20Mass%20Transfer/pdf/M6/Student_Slides_M6.pdf

The Prandtl number (Pr = ν / α) is defined as the dimensionless ratio between kinematic viscosity (ν) and thermal diffusivity, α = k / (ρ·cp); where k stands for thermal conductivity, ρ stands for density, and cp is the (isobaric) specific heat capacity.

Prandtl number signifies the thickness of thermal boundary layer and thickness of hydrodyanamic boundary layer, depending on whether it is equal to one, or more than one or less than one. If it is equal to one, it signifies that thickness of thermal boundary layer is equal to that of velocity boundary layer. Hence it the ratio of momentum diffusivity to thermal diffusivity.

Significance -

If Prandtl number is small, it tells us that thermal diffusion is dominant in comparison to momentum diffusion. That is for a given fluid flow problem, the flow conditions remaining the same, if we want higher heat transfer rate we have to use a fluid that has lower Prandtl number.

Higher values of Prandtl number strongly affects not only the heat transfer, but also the temperature distribution.