What does "5k1" in the given schematic mean?
Resistors are often marked using the letter k (or letter M) instead of the decimal separator, so 5k1 means 5.1k\$\Omega\$ or 5100\$\Omega\$, while for example 1M2 means 1.2M\$\Omega\$.

A unique letter for each type of element can substitute for a decimal point in small values. For example, 6R8 is used to indicate a 6.8 \$\Omega\$ resistor. Similarly, the letter V substitutes for a decimal point in voltages such as 3V3 for 3.3 volts.

That same marking style is often used on ceramic or polyester capacitors, where for example "2n2" stands for 2.2nF.

About the diode, that's not just a B, there's written "BAT46" that is a specific kind of diode. A datasheet from Vishay can be found here.