The "most acidic form" is the one in which all ionizable groups in the molecule are protonated. It is the form in which the molecule would exist in a very acidic solution, say pH 1 or less.
In that state, the amino group on serine would be in the form of NH3+. The carboxy-terminal carboxyl group would also be protonated, and so would be in the form --COOH. Since the side-chain of histidine also has a protonatable nitrogen, that nitrogen would also be protonated and would carry another positive charge.
In peptides, there is some degree of resonance at each peptide bond. So, the oxygens of the peptide bonds might be negatively charged with a double bond between the C and the N of the peptide bond.
In this peptide, serine's side chain and histidine's side chain are both hydrophilic while proline's side chain is hydrophobic.