Horse "foaming at the mouth" What does that mean when you see it?

[Linny]

RA seems to handle things pretty well. She generally schools in the paddock a day or two before racing but never seems to show signs of stress. She looks around, chews the bit and generally chills out.
Froth is not a concern. My mare goes in a rubber bit with apple flavoring in it and slobbers up a storm when were have a good lesson. She's a happy and healthy mare who like RA just looks around with ears pricked but never seems too worried by anything she sees.