This means that you will need to replace the drum soon. Order a new drum and when the light becomes steady or the print quality becomes poor, replace the drum. The drum should last ~12,000 pages. The part number is DR350. You may be able to order this from your Office Supply store; otherwise try the Brother Mall site: http://www.brothermall.com/Accessories/AccessoryDetail.aspx?AccessID=DR350&AspxAutoDetectCookieSupport=1
The procedure is similar to replacing the toner but instead of putting a new thing of toner inside the drum, you remove the toner and put it in the new drum. Open the front cover and pull out the drum and toner assembly. Place on newspaper or other disposable material (toner may spill). Push down the blue lever to unlock the toner cartridge. Remove the cartridge from the drum. Unpack the new drum (keep it out of the light until ready to replace the drum). Push the toner cartridge into the new drum until it locks into place; the blue lever should lift automatically. Put the assembly back in the printer. Clean the printer as recommended in the manual while it is open; this includes cleaning the corona wire before replacing the drum assembly.
After replacing the drum, reset the drum counter: with the printer on and the drum light blinking, open the front cover and press and hold the GO button until all of the LEDs light up (about 4 seconds). Then release the GO button and close the cover. Check that the drum light is off. (The manual says you should do the reset first but I tend to avoid this in case of distractions.)
I hope this helps.