Those symbols indicate which cellular network standard your mobile data connection is currently using, which in turn determines your maximum connection speed.
H stands for HSPA, or High Speed Packet Access, with a maximum theoretical speed of 14 Mb/s, or 42 Mb/s with HSDPA (High Speed Downlink Packet Access).
H+ stands for HSPA+, or Evolved HSPA, with a maximum theoretical speed of 168 Mb/s.
Both of the above are part of the 3.5G family of cellular network standards.
Other possible symbols you might see include:
G, which stands for GPRS, or General Packet Radio Service, a 2G standard with a maximum theoretical speed of 114 kb/s.
E, which stands for EDGE, or Enhanced Data Rates for GSM Evolution, a 2.5G standard with a maximum theoretical speed of 236 kb/s.
3G, which stands for UMTS, or Universal Mobile Telecommunications System, a 3G standard with a maximum theoretical speed of 384 kb/s, although most 3G networks have been upgraded with HSPA and related standards and now support much faster speeds.
LTE, which stands for Long-Term Evolution, technically a 3.5G standard, although marketed as 4G, with a maximum theoretical speed of 300 Mb/s.
4G, which stands for LTE Advanced, a 4G standard with a maximum theoretical speed of several Gb/s, although I don't believe speeds that high are actually available on commercial networks yet.
In all cases, the maximum speed you can actually achieve will depend on the capabilities of your phone and the mobile network, your distance from the cell tower, any limitations imposed by your carrier, as well as a number of other factors. Also, in most cases the maximum speed applies to the downlink (i.e. information transferred from the network to your phone); uplink (information transferred from your phone to the network) speeds are often significantly lower.
For more information on the various icons in the status bar, see What do the icons on my phone mean? at WindowsPhone.com.